Haizhou Xue

Ionization-induced annealing of pre-existing defects in silicon carbide

A long-standing objective in materials research is to effectively heal fabrication defects or to remove pre-existing or environmentally induced damage in materials. Silicon carbide (SiC) is a fascinating wide-band gap semiconductor for high-temperature, high-power and high-frequency applications. Its high corrosion and radiation resistance makes it a key refractory/structural material with great potential for extremely harsh radiation environments. Here we show that the energy transferred to the electron system of SiC by energetic ions via inelastic ionization can effectively anneal pre-existing defects and restore the structural order. The threshold determined for this recovery process reveals that it can be activated by 750 and 850 keV Si and C self-ions, respectively. The results conveyed here can contribute to SiC-based device fabrication by providing a room-temperature approach to repair atomic lattice structures, and to SiC performance prediction as either a functional material for device applications or a structural material for high-radiation environments.

Synergy of elastic and inelastic energy loss on ion track formation in SrTiO3

While the interaction of energetic ions with solids is well known to result in inelastic energy loss to electrons and elastic energy loss to atomic nuclei in the solid, the coupled effects of these energy losses on defect production, nanostructure evolution and phase transformations in ionic and covalently bonded materials are complex and not well understood due to dependencies on electron-electron scattering processes, electron-phonon coupling, localized electronic excitations, diffusivity of charged defects, and solid-state radiolysis. Here we show that a colossal synergy occurs between inelastic energy loss and pre-existing atomic defects created by elastic energy loss in single crystal strontium titanate (SrTiO3), resulting in the formation of nanometer-sized amorphous tracks, but only in the narrow region with pre-existing defects. These defects locally decrease the electronic and atomic thermal conductivities and increase electron-phonon coupling, which locally increase the intensity of the thermal spike for each ion. This work identifies a major gap in understanding on the role of defects in electronic energy dissipation and electron-phonon coupling; it also provides insights for creating novel interfaces and nanostructures to functionalize thin film structures, including tunable electronic, ionic, magnetic and optical properties.

Local Structure and Short-Range Order in a NiCoCr Solid Solution Alloy

Multielement solid solution alloys are intrinsically disordered on the atomic scale, and many of their advanced properties originate from the local structural characteristics. The local structure of a NiCoCr solid solution alloy is measured with x-ray or neutron total scattering and extended x-ray absorption fine structure (EXAFS) techniques. The atomic pair distribution function analysis does not exhibit an observable structural distortion. However, an EXAFS analysis suggests that the Cr atoms are favorably bonded with Ni and Co in the solid solution alloys. This short-range order (SRO) may make an important contribution to the low values of the electrical and thermal conductivities of the Cr-alloyed solid solutions. In addition, an EXAFS analysis of Ni ion irradiated samples reveals that the degree of SRO in NiCoCr alloys is enhanced after irradiation.

Electronic stopping powers for heavy ions in SiC and SiO2

Accurate information on electronic stopping power is fundamental for broad advances in materials science, electronic industry, space exploration, and sustainable energy technologies. In the case of slow heavy ions in light targets, current codes and models provide significantly inconsistent predictions, among which the Stopping and Range of Ions in Matter (SRIM) code is the most commonly used one. Experimental evidence, however, has demonstrated considerable errors in the predicted ion and damage profiles based on SRIM stopping powers. In this work, electronic stopping powers for Cl, Br, I, and Au ions are experimentally determined in two important functional materials, SiC and SiO2, based on a single ion technique, and new electronic stopping power values are derived over the energy regime from 0 to 15 MeV, where large deviations from the SRIM predictions are observed. As an experimental validation, Rutherford backscattering spectrometry (RBS) and secondary ion mass spectrometry (SIMS) are utilized to me...

Effects of chemical alternation on damage accumulation in concentrated solid-solution alloys

Single-phase concentrated solid-solution alloys (SP-CSAs) have recently gained unprecedented attention due to their promising properties. To understand effects of alloying elements on irradiation-induced defect production, recombination and evolution, an integrated study of ion irradiation, ion beam analysis and atomistic simulations are carried out on a unique set of model crystals with increasing chemical complexity, from pure Ni to Ni80Fe20, Ni50Fe50, and Ni80Cr20 binaries, and to a more complex Ni40Fe40Cr20 alloy. Both experimental and simulation results suggest that the binary and ternary alloys exhibit higher radiation resistance than elemental Ni. The modeling work predicts that Ni40Fe40Cr20 has the best radiation tolerance, with the number of surviving Frenkel pairs being factors of 2.0 and 1.4 lower than pure Ni and the 80:20 binary alloys, respectively. While the reduced defect mobility in SP-CSAs is identified as a general mechanism leading to slower growth of large defect clusters, the effect of specific alloying elements on suppression of damage accumulation is clearly demonstrated. This work suggests that concentrated solid-solution provides an effective way to enhance radiation tolerance by creating elemental alternation at the atomic level. The demonstrated chemical effects on defect dynamics may inspire new design principles of radiation-tolerant structural alloys for advanced energy systems.

In-cascade ionization effects on defect production in 3C silicon carbide*

ABSTRACT Understanding how energy deposited in electronic and atomic subsystems may affect defect dynamics is a long-standing fundamental challenge in materials research. The coupling of displacement cascades and in-cascade ionization-induced annealing are investigated in silicon carbide (SiC). A delayed damage accumulation under ion irradiation is revealed with a linear dependence as a function of both increasing ionization and increasing ratio of electronic to nuclear energy deposition. An in-cascade healing mechanism is suggested with a low threshold value of electronic energy loss (∼1.0 keV nm−1). The in-cascade ionization effects must be considered in predicting radiation performance of SiC. GRAPHICAL ABSTRACT IMPACT STATEMENT A considerable impact of ionization-induced in-cascade defect annealing is presented. A surprisingly low threshold of electronic energy loss is revealed.

X-ray absorption investigation of local structural disorder in Ni1-xFex (x=0.10, 0.20, 0.35, and 0.50) alloys

Defect energetics in structural materials has long been recognized to be affected by specific alloy compositions. Local structural distortion greatly affects the physical properties and performance of alloys. To reveal the atomic-level lattice distortion, the local structures of Ni and Fe in Ni1-xFex (x = 0.10, 0.20, 0.35 and 0.50) solid solution alloys were measured with extended X-ray absorption fine structure (EXAFS) technique. The EXAFS measurements have revealed that the bond length of Fe with surrounding atoms is 0.01–0.02 A larger than that of Ni with its neighbors in the alloys. Both the lattice constant and the interatomic distance of the nearest neighbors increase with the addition of Fe content in the solid solutions. The local bonding environments in Ni1-xFex alloys were also calculated from ab initio and compared with the experimental results.